PTEKIDOPUYTES 



141 



are spoken of as "leaf veins." Large working leaves and 

 a vascular system, therefore, belong together and appear 

 together; and the vascular plants are also the plants with 

 leafy sporophytes. 



(4) The leaf. — Leaves are devices for spreading out 

 green tissue to the light, and in the Ferns they are usually 

 large, ^here is a stalk-like portion {petiole) which rises 

 from the subterranean stem, and a broad expanded portion 

 {Made) exposed to the light and air (Fig. 118). In Ferns 

 the blade is usually much branched, being cut up into 

 segments of various sizes and forms. 



The essential structure consists of an expansion of 

 green tissue {77ieso2)liyll)^ through which strands of the 

 vascular system {veins) branch, forming a supporting 

 framework, and over all a compact layer of protecting 

 cells {epidermis). A surface 

 view of the epidermis shows 

 that it is pierced by numer- 

 ous peculiar pores, called 

 stomata., meaning " mouths." 

 The surface view of a stoma 

 shows two crescentic cells 

 {guard cells) in contact at 

 the ends and leaving be- 

 tween them a lens-shaped 

 opening (Fig. 121). 



A cross-section through 

 a leaf gives a good view of 

 the three regions (Fig. 122). 

 Above and below is the col- 

 orless e23idermis, pierced 

 here and there by stomata ; 

 between the epidermal lay- 

 ers the cells of tlie mesopliyll are packed; and among 

 the mesophyll cells there may be seen here and tliere tlie 

 cut ends of the veins. The leaf is usually a dorsiventral 



Fig. 121. Some epidermal cells from 

 leaf of Pteris, sjhowiiifr the inter- 

 locking walls and three stomata, the 

 guard cells containing chloroplasts. 

 —Land. 



